chore(main): release 4.0.0 (#372)

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md
Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>